This invention relates to the removal of oxygen from an aqueous solution. More particularly, the invention concerns a method for lowering the oxygen content of an aqueous polymer solution so as to prevent oxidative degradation of the polymer solution.
Removal of oxygen from certain polymer systems used in enhanced oil recovery processes is mandatory, if such chemical systems are to be injected in moderate to high temperature reservoirs. Deoxygenation of aqueous polymer solutions containing partially hydrolyzed polyacrylamides, polysaccharides, and other synthetic high temperature polymers prevents oxidative degradation, especially at elevated temperatures. An oxygen level of about 1 ppb or less in polyacrylamide solutions has been reported to be necessary to inhibit oxidative degradation at high temperatures. Please see, Ryles, R. G. "Elevated Temperature Testing of Mobility Control Reagents," SPE Paper No. 12008 Presented at the 58th Annual Technical Conference of the Society of Petroleum Engineers, San Francisco, Oct. 5-8, 1983.
Oxygen scavenging is usually accomplished by adding oxygen scavenging chemicals into the chemical solution. For example, sulfur-containing antioxidants, water-soluble alcohols or glycols and thiourea are used as described in U.S. Pat. No. 4,218,327. Amine bisulfite scavengers are described in U.S. Pat. No. 3,976,593, and aldehyde scavengers are disclosed in U.S. Pat. No. 3,800,877. A number of other oxygen scavenging chemicals have been disclosed in the patent literature.
For best results and to avoid chemical incompatibility problems between the oxygen scavengers and hardness (Ca.sup.++) ions found in formation brine, and also to minimize free radical generation by the action of chemical scavengers, it is desirable to use as small a quantity of scavengers as possible. To keep the amount of these additives to a minimum, one must remove as much oxygen as possible from the oil recovery system prior to adding oxygen scavengers. This is accomplished in part by purging or blanketing aqueous solutions of polymers with nitrogen in tanks. However, blanketing solutions with nitrogen is not effective in reducing oxygen levels down to very low levels. Bubbling N.sub.2 through aqueous surfactant solutions or through polymer solutions prepared using stock polymer solutions in emulsion form creates foaming in the preparation tank which is detrimental to the solution preparation process.
U.S. Pat. No. 3,751,879 discloses a tubular exchanger for reducing the dissolved gas concentration in a liquid, wherein the dissolved gas passes through a gas permeable membrane. U.S. Pat. No. 4,268,279 discloses a gas transfer process with a hollow fiber membrane wherein gaseous components contained in two different liquids are transferred to the opposite liquid with a hollow fiber membrane, preferably a fluorinated polyolefin. Microporous polyethylene hollow fibers and a process of preparing them are disclosed in U.S. Pat. No. 4,020,230. Similar porous hollow fibers made of polypropylene are described in U.S. Pat. No. 4,055,696.
A process for decreasing the oxygen content of a fluid is described in U.S. Pat. No. 4,516,984. This patent discloses using teflon, polysilicon, polypropylene and polyethylene membranes in the shape of tubing to run chemical solutions through a vessel containing an oxygen scavenging solution. The tubing, which is permeable to oxygen, passes oxygen into the oxygen scavenging solution, lowering the oxygen content of the fluid.